RFR: 8292847: Zero: Allow ergonomics to select the GC

[Aleksey Shipilev]

Zero is currently defaulting to Serial GC on all machines, due to the fact that `NeverActAsServerClassMachine` is `true`. It is set in `compiler_globals.pd` under `#if !defined(COMPILER1) && !defined(COMPILER2) && !INCLUDE_JVMCI`. `NeverActAsServerClassMachine` mostly affects compiler ergonomics, which Zero does not need, but it also affects the selection of default GC.

After [JDK-8256497](https://bugs.openjdk.org/browse/JDK-8256497), we can let Zero default to G1. After [JDK-8292329](https://bugs.openjdk.org/browse/JDK-8292329), it would help Zero to use the CDS shared heap.

Additional testing:
 - [x] Linux x86_64 Zero fastdebug, `make bootcycle-images`
 - [x]  Linux x86_64 Zero fastdebug, `tier1` tests (many known failures)

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Commit messages:
 - Reword the comment
 - Fix

Changes: https://git.openjdk.org/jdk/pull/9994/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9994&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292847
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9994.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9994/head:pull/9994

PR: https://git.openjdk.org/jdk/pull/9994